1. Field of the Invention
The present invention relates to a light-emitting material, a scintillator containing the light-emitting material, an x-ray detector equipped with the scintillator, an image display device using the light-emitting material, and a light source using the light-emitting material.
2. Related Art
In recent years, there has been a growing demand for a fast-response and higher-sensitivity X-ray detector as the speed of X-ray CT (Computed Tomography) has become faster and the definition of X-ray CT has become higher. Currently, an X-ray detector mainly used is composed of a scintillator for converting X-ray into light and a light-receiving element for converting the light into electrical signals. Therefore, in order to realize a fast-response and higher-sensitivity X-ray detector, it is necessary to improve the properties of the scintillator and the emission properties of a light-emitting material constituting the scintillator. In order to realize a fast-response X-ray detector, it is necessary to use a light-emitting material having a short decay time. For example, when a decay time is defined as the time required for emission intensity to decay to 10%, the decay time of the light-emitting material is preferably 10 μs or less. Further, in order to realize a high-sensitivity X-ray detector, it is necessary to use a light-emitting material having a higher luminous efficiency, and at the same time it is preferred that the emission spectrum of the light-emitting material is highly matched with the sensitivity spectrum of a light-detective element used.
Such an x-ray detector currently used mainly has a silicon photodiode as a light-detective element. The spectral sensitivity distribution of the silicon photodiode has a peak in the near-infrared region, and therefore the emission wavelength of a light-emitting material used is preferably 650 nm or longer in the region from deep red to near-infrared.
Examples of known scintillators for X-ray detectors include a single crystal scintillator composed of NaI:Tl, a single crystal scintillator composed of CdWO4, and a ceramic scintillator composed of Gd2O2S:Pr. These scintillators each have a decay time of less than 10 μs, and are therefore suitable for use in fast-response X-ray detectors. However, the peak emission wavelengths of these scintillators are 415 nm, 470 nm, and 510 nm, respectively, which are not sufficiently long from the viewpoint of matching with a silicon photodiode. In addition to them, a ceramic scintillator composed of (Y,Gd)2O3:Eu is also known. This ceramic scintillator has a peak emission wavelength of 610 nm which is longer than those of the above-mentioned scintillators but is not yet sufficiently long from the viewpoint of matching with a silicon photodiode. In addition, the ceramic scintillator composed of (Y,Gd)2O3:Eu has a decay time of 1 ms or longer, and is therefore not suitable for use in fast-response X-ray detectors. It is to be noted that the symbol (Y,Gd) represents a mixed crystal of Y (yttrium) and Gd (gadolinium).
Here, use of Eu2+, which emits light by an allowed transition, as a luminescent center can be considered as one means for obtaining a light-emitting material having a short decay time. A list of light-emitting materials using Eu2+ as a luminescent center is shown in a literature by P. Dorenbos (see J. Lumin., 104 (2003), pp. 239-260). In this list, light-emitting materials having an emission wavelength of 650 nm or longer are limited to chemically-unstable materials likely to react with water such as CaO and CaS and nitrides difficult to be synthesized.
Meanwhile, a phosphor is used as an important material exerting an influence upon the performance of devices used in various fields such as illumination, displays, and medical devices. The term “phosphor” used herein refers to one of light-emitting materials, and in the following description, the term “phosphor” can be used synonymously with the term “light-emitting material”. For example, in the field of illumination, a phosphor exerts a great influence upon the performance, such as efficiency and color rendering properties, of fluorescent lumps and devices such as white LEDs which have been significantly technologically advanced in recent years. In the field of displays, emission properties of a phosphor greatly influence the image display performance of self-luminous display devices such as plasma display panels (PDPs) and field emission displays (FEDs). Further, in the case of nonluminous displays such as liquid crystal displays, a backlight exerts a great influence upon display performance, and therefore it is no exaggeration to say that a phosphor used for the backlight exerts a great influence upon the image display performance of liquid crystal displays. As has been described above, since a fluorescent material is used as an important material in various devices utilizing light emission and exerts an influence upon the performance of these devices, there has been a growing demand for a phosphor having improved emission properties.
In order to meet the demand, various light-emitting materials have been developed up to now and some of them are practically used as phosphors. Major phosphors are shown in various literatures such as PHOSPHOR HANDBOOK Edited by S. Shionoya and W. M. Yen, pp. 391-394, pp. 511-520, etc., CRC Press (1999), and the like, and some of them will be described below. A typical example of a display device excited by an electron beam includes a direct-view-type cathode ray tube (CRT). In such a direct-view-type CRT, ZnS:Ag,Cl or ZnS:Ag,Al is typically used as a blue light-emitting phosphor, ZnS:Cu,Al is typically used as a green light-emitting phosphor, and Y2O2S:Eu is typically used as a red light-emitting phosphor. These phosphors emit light highly efficiently when excited by a relatively low energy-density electron beam as in the case of a direct-view-type CRT. However, in a case where these phosphors are excited by a high energy-density electron beam, their luminous efficiency is lowered due to saturation. Therefore, in the case of a device excited by a high energy-density electron beam such as a projection-type CRT, phosphors which emit high-luminance light under excitation by a high energy-density electron beam, such as Y2SiO5:Tb as a green light-emitting phosphor and Y2O3:Eu as a red light-emitting phosphor, are used. However, there is no known blue light-emitting phosphor which emits light having a higher luminance than light emitted from a phosphor represented by ZnS and an excellent emission color under excitation by a high energy-density electron beam.
In the field of displays, flat panel displays have come to be widely used instead of CRTs in recent years. Among various flat panel displays excited by an electron beam, FEDs and SEDs (Surface-Conduction Electron-Emitter Displays) are receiving attention. The fluorescent materials of such FEDs and SEDs are excited by a higher energy-density electron beam than the phosphors of direct-view-type CRTs. Therefore, phosphors which are less likely to allow luminance saturation to occur are required for FEDs and SEDs, and particularly the advent of a blue light-emitting phosphor which emits light having a higher luminance than light emitted from a phosphor represented by ZnS under excitation conditions of FED or SED has been awaited. As a backlight light source for liquid crystal displays, a cold cathode fluorescent lamp is most commonly used. A typical example of a phosphor for use in the cold cathode fluorescent lamp includes a mixture of a blue light-emitting material represented by BaMgAl10O17:Eu (BAM), a green light-emitting material represented by LaPO4:Ce,Tb, and a red light-emitting material represented by Y2O3:Eu.
On the other hand, an attempt has been recently made to use a white LED light source as a backlight light source. A typical example of a white LED light source includes a combination of a blue light-emitting diode (LED) and a yellow light-emitting phosphor represented by (Y,Gd)3Al5O12:Ce. In order to satisfy a requirement to widen the color reproduction range of a liquid crystal display, it is preferred that when a light source is used together with red, blue, and green color filters to measure the chromaticity coordinates of red, blue, and green light, the area of a triangle formed on a chromaticity diagram by connecting the chromaticity coordinates of red, blue, and green light is made larger. Therefore, a phosphor for use in a backlight light source is required to have an emission spectrum satisfying the requirement, and the advent of a phosphor which can widen the color reproduction range of a liquid crystal display has been awaited. Further, from the viewpoint of production, the advent of a phosphor which can emit white light by itself has been awaited because the spectrum of a light source can be more easily controlled when white light is obtained by using only one kind of phosphor than when white light is obtained by mixing two or more phosphors. As has been described above, various phosphors have already been known, but the advent of phosphors having improved emission properties has been awaited to satisfy requirements varying depending on their purposes of use.
As has been described above, any conventional scintillators do not satisfy both the requirements of a short decay time and a long emission wavelength.
Further, as has been described above, the properties of the well-known light-emitting materials are not yet sufficient for many purposes of use, and therefore the advent of novel phosphors such as a blue light-emitting phosphor which emits light having a higher luminance than light emitted from a phosphor represented by ZnS and an excellent emission color under excitation by a high energy-density electron beam, and a white phosphor capable of realizing a backlight light source which can widen the color reproduction range of a liquid crystal display when used together with red, green, and blue color filters has been awaited.